Handle filename variations for prebuilt_etc in bp2build

Bp2build now handles these cases:

-filename is specified as a property
-if filename is not specified and filename_from_src = true,
	filename is set as the src
-filename defaults to the module name otherwise

Bug: 228353188
Test: bp2build/prebuilt_etc_conversion_test.go
Change-Id: Ia4c9b2c89f100e4ed9363dc3ba20ea501b776216