releasetools: Add PayloadSigner class.

Create a wrapper class that handles the payload signing, which unifies
the paths with and without external signer. Also add tests for the newly
added class.

The test keys under testdata/ are created with the script in
development/tools/make_key. testdata/testkey_with_passwd.pk8 uses
password "foo".

Test: python -m unittest test_ota_from_target_files
Test: Get identical A/B OTA packages w/ and w/o the CL.
Change-Id: Ic770aec726498a3babb88ec509985e7f1210fb18

tools/releasetools/testdata/payload_signer.sh

@@ -0,0 +1,4 @@
+#!/bin/sh
+
+# The script will be called with 'payload_signer.sh <key> -in <input> -out <output>'.
+openssl pkeyutl -sign -keyform DER -inkey $1 -pkeyopt digest:sha256 -in $3 -out $5

tools/releasetools/testdata/sigfile.bin

@@ -0,0 +1 @@
+<07>Q<EFBFBD><51>ܢ<EFBFBD><DCA2><EFBFBD><EFBFBD>Gp<47><70><EFBFBD><EFBFBD><10><><EFBFBD><EFBFBD>'[4K<34>L<>c

tools/releasetools/testdata/signed-sigfile.bin

@@ -0,0 +1,2 @@
+R<EFBFBD><13>&<26>E<EFBFBD>s<EFBFBD>%<25>?<3F>|<7C><>&<26>̀<EFBFBD>zbS<62>A[<5B>tq<74><1B>WKґl<14><><EFBFBD><EFBFBD><EFBFBD>s<EFBFBD><73>~Fc<46>	<09>`<60><><EFBFBD><EFBFBD>#
+T{<7B><>۽F<DBBD><06><><EFBFBD>x<EFBFBD><78>1<EFBFBD>6̋=Q<><51><EFBFBD>V<EFBFBD>^T߰<54>xX<78><58>/<2F>#<23><>I'<27><17>tcLp<><70><EFBFBD><06><><EFBFBD>ovzђR<D192>:<3A><>W<EFBFBD>9(<28><>26<32>̬<EFBFBD>b<>BP1<50>6<16>n<EFBFBD>߱Q<DFB1>C<EFBFBD>g<EFBFBD>h;r<><72>O}%Ľ<><C4BD><EFBFBD><18>o6<14><>d<13><><14><><EFBFBD><07>2Y`<60><>ۼ<><DBBC>_R<5F>OrCa,<2C>I"n(`<60><>n<EFBFBD><6E><EFBFBD>bai<61><69>Ũ<EFBFBD><1A>S<EFBFBD><53><EFBFBD>)k<>O[`6c<36>e

tools/releasetools/testdata/testkey.x509.pem

@@ -0,0 +1,24 @@
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+-----END CERTIFICATE-----

tools/releasetools/testdata/testkey_with_passwd.x509.pem

@@ -0,0 +1,24 @@
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