Write AB_OTA_POSTINSTALL_CONFIG to META/postinstall_config.txt.

The config file will be passed to delta_generator to add the postinstall
field in the update payload.

Bug: 24537566
Change-Id: I06b8d9d0b2c37f9a3e7a844ac4d9e2eead8cd4d2
TEST: make dist
1 file changed