Firstly export the certificate: Proxy > Options > Import/Export CA certificate > DER Format or download this via http://burp
- put the certificate in android, access with a file manager, and install for "VPN and apps" (can be needed to rename the extension 'der' to 'cer')
- since android 7 (nougat), Apps that target API Level 24 and above no longer trust user or admin-added CAs for secure connections, by default. [1]