Add .apex suffix to apex files during validation

Previously, the apex file being installed was moved into the staging
directory as same name provided during installation, due to which, when
the source apex file of installation did not have .apex suffix in its
file name, it failed validation in apexd. Now, if an apex file does not
have proper suffix, it will be concatenated during validation in
PackageInstallerSession.

Bug: 124837227
Test: atest CtsStagedInstallHostTestCases
Test: atest StagedInstallTest#testInstallStagedApexWithoutApexSuffix
Change-Id: Ice41f601dc42736ca0cdf2bad0817f0c00f50648