Add a new property trim_extension for gensrcs

In order to provide a more flexible ability for gensrcs's output, due to
current output_extension property will only allow to replace the string
after the last "." of input, in order to handle input with multiple dot
in file name, provide a way to trim the suffix of input string.

Bug: 335536003
Test: cd build/soong/genrule ; go test -run TestGenSrcs
Test: cd build/soong/genrule ; go test -run TestGenSrcsWithTrimExtAndOutpuExtension
Test: cd build/soong/genrule ; go test -run TestGenSrcsWithTrimExtButNoOutpuExtension
Test: cd build/soong/genrule ; go test -run TestGenSrcsWithOutpuExtension
Test: cd build/soong/android ; go test -run TestPathsForModuleSrc

Change-Id: I033bbe1d225f207f0f6bdc140df308884f214b51